The company said it had shipped gasoline and other supplies from Texas to fill the production gap.

"There should be very minimal market impact, if any, resulting from these required repairs," Scott Folwarkow, Valero's governmental affairs director, said in a Thursday letter to the California Energy Commission.Rob Schlichting, an Energy Commission spokesman, said, "Valero appears to have covered [the shortfall] with outside supplies."

Folwarkow's letter said that "in preparation for this required repair, we shipped 110,000 barrels of [reformulated] gasoline into California from our Corpus Christi refinery." That shipment arrived in California on Thursday.

Valero said that daily gasoline output would fall by 20,000 barrels, or 840,000 gallons, while it shuts down a naphtha reformer in order to fix a steam leak in a flare that could otherwise emit excessive smoke. The shutdown began Friday and is expected to last for 10 days. The company did not say specifically when the maintenance problem was identified.